Semifinals: The Big Four and Wozniacki's shot

Saturday will feature all four singles semifinals. Novak Djokovic and Roger Federer will kick things off at noon on Arthur Ashe Stadium, followed by Andy Murray-Rafael Nadal (not before 2:30 p.m. on Ashe), Sam Stosur-Angelique Kerber (6 p.m., Grandstand) and Serena Williams-Caroline Wozniacki (8 p.m., Ashe). Here's a look at each match:

MEN'S SEMIFINALS

Roger Federer (left) is the only player to beat Novak Djokovic this year in a completed match. (Icon SMI)

Novak Djokovic vs. Roger Federer: The last time these two met, Federer snapped Djokovic's 43-match unbeaten streak with a 7-6 (5), 6-3, 3-6, 7-6 (5) victory in the semifinals at the French Open. The win meant a lot to Federer, who memorably wagged his index finger after firing an ace on match point. Federer had been somewhat overlooked leading up to Roland Garros, with the year's storylines centered on Djokovic's streak, Rafael Nadal's inability to solve the Djokovic riddle and, to a lesser extent, Andy Murray's post-Australian Open slump.

The Swiss came into the U.S. Open in a similar position, as questions about his form swirled. Well, he's shut everyone up now, cruising through the draw (much as he did in Paris) by dropping only one set in five matches. Djokovic is still Djokovic, though, and he too has lost only one set, despite some ever-so-slight sluggishness that has popped up now and then. He's also 3-0 against Federer on hardcourts this year.

The key to the match will be Federer's serve. His serve was untouchable in Paris and his ability to hold confidently gave him the freedom to attack freely on Djokovic's serve. Never underestimate Fed's ability to get himself up for matches against Djokovic. PREDICTION: Federer in four.

Rafael Nadal vs. Andy Murray: This is Murray's fourth Slam semifinal of the year, making him the seventh player to accomplish the feat (Djokovic also joined the club this year). Nadal is 12-4 against Murray, but when the Scot has come out on top (or when he's been able to push the Spaniard), it's been on hardcourts. Nadal ended Murray's campaign at the last two majors, and despite my early-tournament concerns about Rafa, it's looking like he'll do it again.

The defending champion's movement and footwork have steadily improved over the course of the fortnight, and he was absolutely dominant against a fatigued Andy Roddick on Friday. Murray sputtered a bit in a four-set win against John Isner. I'm not one to think that form matters too much when it comes to the Big Four. They just know how to play each other and they seem to find their game as the familiarity of the matchup kicks in. Keep an eye on Murray's serving stats. He's going to need a big serving day. PREDICTION: Nadal in three.

WOMEN'S SEMIFINALS

Caroline Wozniacki can silence her critics with a victory against Serena Williams. (EPA)

Serena Williams vs. Caroline Wozniacki: I've been anticipating two matches with Williams since she announced her return to the Tour: Serena against an in-form Petra Kvitova (still waiting on that one) and Serena against Wozniacki. This is a career-defining moment for Wozniacki. She's often described as "a paper No. 1," a player who takes advantage of the WTA rankings system by playing as many Tour-level tournaments as possible to accumulate points, only to come up short in the the Slams. The criticism is unfair (you still have to win matches to get those points and Wozniacki wins a whole lot), but we want our No. 1 players to be great, and in this day and age Slam success is the measure of greatness. So here's her shot.

If Wozniacki beats Serena -- and I believe she can -- the Dane will have knocked off the player whom most consider the best in the world at a Grand Slam. With that, the criticism stops. It's a unique situation to be in, as Wozniacki is the one who has nothing to lose here. Even the U.S. Open's official Twitter account has already put Serena into the final. The two have played only one completed match, in 2009 in Sydney, and Wozniacki actually had match points before losing 6-7 (5), 6-3, 7-6 (3). If Wozniacki doesn't serve well, then Serena will be eating her second serve for dinner. But Wozniacki has the legs and the competitive fire to frustrate Serena, who needs to be patient and work points before ripping winners. If Serena starts racking up unforced errors, we'll have ourselves a match. PREDICTION: Williams in three.

Sam Stosur vs. Angelique Kerber: There's no way around it: Angelique Kerber is a shocking semifinalist. She's 23 years old, ranked no. 93 in the world and Stosur admits that she's never seen her play and knows nothing about her other than the fact that she's left-handed. That fact alone might give Stosur pause, because a lefty has eliminated her from two Slams this year: Kvitova in Melbourne and Melinda Czink in Wimbledon.

Kerber is of the modern German mold: She's strong, she hits hard and her movement can, at times, be an issue. In the midst of a virtually nonexistent year, and on the heels of her first-round loss to Laura Robson at Wimbledon, Kerber spent some time training with Andrea Petkovic in Germany. Petkovic's positive reinforcement might have helped, as Kerber made a strong run (through a weak field) to the semifinals in Dallas two weeks ago. Being the unknown quantity could benefit her in this matchup.

But this is obviously Stosur's match to win, and she has a game that can take an opponent completely out of the match. If the No. 9 seed keeps doing what she's done all tournament -- serve big, strike her forehand and not fear hitting her backhand flat up the line -- she'll book a spot in her second career Grand Slam final. PREDICTION: Stosur in two.

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